Technical Studies

Natural rivers are often compound channels constituted by main channel and flood-plains, therefore, the flow conditions are very complicated during the floods. The characteristics of Taiwan’s rivers include steep slope and high flow velocity. Channel migration and bed erosion/deposition often happen at the lower-middle reaches. Under the global climate change, the variation of rainfall intensity, rainfall duration, rainfall distribution and flow discharge will result in extension of the period of low water level or increase of the main channel flow. Therefore, reconsideration of assessment of channel stability and protection standard of embankment is necessary. Many of rivers in the Western Taiwan have the problems of channel migration. The flow in meandering river is affected by centripetal force, which results in secondary flow and super-elevation along concave bank. According to analysis of annual aerial-photographs, the behavior of channel migration is evident. By considering the seriousness of the channel migration and the representation, Tsengwen River is selected as the target this year. The study adopts the assessment of future rainfall due to clime change in Tsengwen River predicted by NCDR, and uses SOBEK rainfall-runoff model to estimate the associated discharge. The results show under climate change the flood magnitudes may reach the return period of 100-yr to 200-yr, which exceed the design flood in the lower-middle reaches of Tsengwen River. Therefore, this study discuss the stability of the compound channel and the process of channel migration of Tsengwen River by collecting basic data including those associated with the climate change, and using 1-D and 2-D sediment transport models. Finally, the procedure for evaluating the channel migration problem under the increase of main-channel flow and the feasible countermeasures is proposed.